Helicopters and analogous aircraft are fitted with rotors that are driven in rotation by turbine engines associated therewith. In general terms, these turbine engines comprise a gas compressor, a fuel combustion chamber, and a turbine arranged in succession. The compressor generates air under pressure, which is then heated in the combustion chamber so as to drive the turbine in rotation. When the turbine engine is running on its own, the compressor is driven in rotation by the turbine. Turbine engines may be of the coupled turbine type, in which the turbine is coupled in rotation with the compressor by a working shaft, or of the free turbine type, with the compressor and the turbine being mounted on separate shafts. A problem arises concerning driving the turbine engine during starting until a drive threshold is reached at which the turbine is rotating sufficiently to enable the turbine engine to operate on its own.
It is known to fit the turbine engine with an auxiliary electric motor or “starter” in order to drive the compressor until said rotary drive threshold is reached for the turbine. The starter drives the compressor mechanically until a suitable compression threshold is reached at which fuel can be injected and the ignition circuit of the turbine engine can be operated. Thereafter, the starter continues to be operated until the turbine reaches a speed of rotation that is sufficient to allow the turbine engine to operate on its own. The starter is powered during starting by a source of electrical energy that is on board or that is external to the helicopter. For on-board electrical energy sources, they are generally constituted by an on-board battery and/or an electricity generator that serves to power the starter while it is starting the turbine engine.
A difficulty to be overcome lies in the need to deliver sufficient energy to the starter to drive the compressor against the opposing torque from the turbine. The torque delivered by the starter must remain greater than said opposing torque in order to achieve sufficient acceleration in the speed at which the compressor is driven, until the turbine engine starts operating on its own. The opposing torque from the turbine is particularly great at the beginning of starting the turbine engine, in particular prior to reaching said compression threshold suitable for injecting fuel and igniting the turbine engine. Once fuel is being injected and after the turbine engine has ignited, the starter and the turbine engine itself both act against the opposing torque so as to provide a force for driving the compressor, which force increases progressively until the turbine engine is operating on its own.
The mechanical torque that the starter is capable of delivering during starting needs to be adapted so as to drive the compressor until the turbine engine is operating on its own. Using on-board batteries for powering the starter cannot be satisfactory, particularly when they are used at so-called “extreme” temperatures or if the batteries are discharged. Batteries constitute a source of limited energy while nevertheless presenting a weight that is large. The batteries are also used for powering other members of the helicopter, which means are used both on the ground prior to and during starting of the turbine engine, and also while in flight. Batteries need to satisfy requirements for operating at low temperatures, in particular because batteries are weaker at low temperatures while the opposing torque from the turbine against setting the compressor into rotation increases at low temperatures. Consequently, it is necessary for on-board batteries to have a comfortable reserve of energy, with the consequent drawback of increasing their weight, and their size, and thus increasing the load that needs to be carried by the helicopter.
Mention is made of document US 2008/0246443 (Doljack), which describes a device for improving an operating stage that generates an electricity consumption peak either in a self-propelled terrestrial vehicle (FIG. 1A), or in a portable electronic appliance (FIGS. 1C-E). That device makes provision, permanently in its circuit (100), for a module (105) of supercapacitors connected in series (120) relative to a charger (110) or a battery (115). For a terrestrial vehicle (100), the voltage appears to be 12 volts (V), and for portable appliances it appears to be 5 V (§0038). The voltage of the module (105) is independent of that of the battery (115).
Document U.S. Pat. No. 6,321,707 (Dunn) describes a direct current (DC) system for starting vehicle engines. The battery voltage of the vehicle is 12 V. An auxiliary energy storage device is installed in the vehicle in optionally-removable manner.